1. Field of the Invention
The present invention relates to a vehicle generator regulating apparatus that controls an output voltage of a vehicle generator.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2002-119098 discloses a regulator for a vehicle generator, which is constituted by a voltage detector circuit, a voltage step-up circuit, and a switching device including a MOS transistor that operates for controlling a field current flowing thorough a field coil of the vehicle generator. The drain of the MOS transistor is connected to an output terminal of the vehicle generator and a positive terminal of a battery being charged by the vehicle generator. The source of the MOS transistor is connected to an end of the field coil. The other end of the field coil is grounded to a vehicle body.
The voltage detector circuit, whose input terminal is connected to the positive terminal of the battery and whose output terminal is connected to the voltage step-up circuit, detects the output voltage of the battery, and produces a drive signal (on/off signal) used for turning on and off the MOS transistor in order to keep the output voltage of the battery at a predetermined value.
The voltage step-up circuit, which is constituted by an oscillator circuit and a charge-pump circuit, produces a high voltage to be applied to a gate of the MOS transistor on the basis of the drive signal received from the voltage detector circuit. The charge-pump circuit performs repeatedly a cycle of charging and discharging multiple capacitors thereof by use of an oscillation signal (periodic pulse signal) outputted from the oscillator circuit for stepping up an input voltage applied thereto and applying the stepped-up voltage to the gate of the MOS transistor.
If the output voltage of the battery falls below the predetermined value, the voltage detector circuit outputs the on signal (drive signal) to the voltage step-up circuit. The charge pump circuit of the voltage step-up circuit, upon receiving the on signal, starts the cycle of charging and discharging the multiple capacitors, thereby applying the high voltage to the gate of the MOS transistor to turn on the MOS transistor. In consequence, the field current starts to flow through the field coil, so that the vehicle generator starts its power generation operation to charge the battery. As a result, the output voltage of the battery increases.
On the other hand, if the output voltage of the battery exceeds the predetermined voltage, the voltage detector circuit outputs the off signal (drive signal) to the voltage step-up circuit. The charge pump circuit of the voltage step-up circuit, upon receiving the off signal, ceases the cycle of charging and discharging the multiple capacitors. As a result, the output voltage of the charge pump decreases rapidly, and the MOS transistor is turned off. In consequence, the field current flowing through the field coil is interrupted, and the vehicle generator stops the power generation operation. As a result, the output voltage of the battery decreases, because the battery is supplying electric power to electric loads.
Through these operations repeatedly performed by the regulator, the output voltage of the battery can be kept at the predetermined value.
In such a regulator, it is necessary for the charge-pump circuit to quickly build up its output voltage so that the MOS transistor can well respond to the drive signal which the voltage detector circuit outputs. The frequency of the oscillation signal (periodic pulse signal) is set at a relatively high frequency, because the build-up speed of the output voltage of the charge pump circuit increases with increase of the frequency of the oscillation signal which is outputted from the oscillator circuit.
Incidentally, a high frequency current resulting from the charging and discharging the multiple capacitors flows into the battery as a noise current. The frequency of the noise current increases with the increase of the frequency of the oscillation signal which the oscillator circuit outputs. Accordingly, if the frequency of the oscillation signal is set at a high value, the chance of the electric loads supplied with electric power from the battery being affected by this noise current becomes high.